1. A microwave spectrometer capable of measuring stratospheric water vapour in Polar regions: VESPA-22
G. Mevi1,2, G. Muscari1, P. P. Bertagnolio1, I. Fiorucci1
1Istituto Nazionale di Geofisica e Vulcanologia
2Università Roma Tre

2. The importance of polar stratospheric water vapour
Stratospheric water vapour regulates cloud formation in the upper troposphere and is involved in the surface temperature increasing measured in the last decades.
(Solomon et al., 2010)
It can be used as a tracer to study the air mass dynamics and the exchanges between troposphere and stratosphere (Holton et al., 1995).
It has an important role in the seasonal ozone loss observed in the polar stratosphere.

3. An indirect measurement: the microwave spectroscopy
We can obtain information about
the vertical concentration profile
of the water vapour by measuring
its thermal emission.
Benefits:
The transition is between well populated states at typical stratospheric temperature
We can take measurements during both day and night
We can neglect the scattering due to aerosol and atmospheric molecules
We can use low cost components to assembly the instrument

4. An indirect measure: the microwave spectroscopy
We can obtain information about
the vertical concentration profile
of the water vapour by measuring
its thermal emission.
Disadvantages:
We need at least 6 hours of integration of our signal to grant a significant
signal to noise ratio
We need an observation site characterized by a dry troposphere to
minimize the signal absorption.

5. Key physical principles behind the measurement
Radiative transfer
Pressure broadening

6. The spectrometer VESPA-22
Central frequency: GHz
Bandwidth: 500 MHz
Frequency resolution: 31 kHz (16384 channels)
No need for cryogenic cooling
Theoretically we can retrieve H2O vertical profile
from 30 to 90 km of altitude

7. The spectrometer VESPA-22
The signal is channelled to the front end electronics by a parabolic mirror
The mirror can rotate to receive radiation from different angles.
The front end is composed by low noise amplifiers, a noise diode to calibrate the signal and two mixers to shift the signal to lower frequencies.
The back end spectrometer converts the signal from the time to the frequency domain.

8. How to get the stratospheric signal: the balanced beams technique
The signal of our interest is very small compared to the tropospheric background signal.
In order to observe the stratospheric emission we compare the emission from two different
beams at different zenital angles and rescale our measurement by a scaling factor.

9. How to get the stratospheric signal: the balanced beams technique
By using this technique we remove gain nonlinearities from the spectrum
Tropospheric opacity can change during the measurement. In order to average several hours of data we need a Scaling Factor to normalize the measured spectra
In order to perform this technique we assume a horizontally homogeneous sky

10. Signal calibration
To calibrate the observed spectra we use the known emission of noise diodes.
We calibrate the noise diodes by observing the emission from two black
body sources at known different temperatures (~77 K and ~300 K).

11. From the measured spectrum to the concentration vertical profile
Water vapour vertical profiles can be obtained from our measurements by using the indirect problem theory
We employ an optimal estimation algorithm to retrieve the profile that has
the largest probability of generating the observed spectrum, given the local climatology
Retrieved vertical H2O profile
Measured spectre
Pressure and Temperature atmospheric profiles
Apriori vertical H2O profile and the related spectre

12. The optimal estimation algorithm
We modify our apriori profile using the spectral information
The Optimal Estimation can be seen as a weighted average of the apriori information
given by local climatology and the new information gathered with the measurement
The matrix A can be used as an estimate of the vertical resolution and
sensibility interval of retrieved profiles

13. A test measurement campaign at Campo Imperatore (AQ)

14. A test measurement campaign at Campo Imperatore (AQ)

15. Conclusions and things to do…
Vespa-22 is capable of retrieving vertical stratospheric water vapour profiles from 30
to 90 km altitude, but signal artifacts can reduce this range.
It can operate automatically, with low level technical assistance and with a temporal
resolution of 2-4 measures/day, depending on weather conditions
We are working on the instrumental setup to minimize signal artifacts
The retrieval algorithm must be improved to reduce the dependency of the km
altitude interval to the water vapour concentration in the troposphere. This can be done
using auxiliary measurements as apriori profile
The collected measurements will be studied to better understand the radiative and
dynamical processes that characterize the polar atmosphere.

16. Thank you for the attention!